The present invention relates generally to microelectromechanical systems processing and fabrication methods, and more particularly, to methods for sealing (i.e., packaging) and backside releasing of microelectromechanical systems (MEMS).
Reliable sealing/packaging of movable micromechanical devices is a very critical and challenging step in commercial development of such devices in industrial environments. The challenging aspect is having a hermetically sealed cap on top of the MEMS device that completely isolates it from the surrounding environment while maintaining the movability for the critical mechanical parts of the structure. In other words, the cap should not be in touch with any of the movable sections of the micromechanical system. Furthermore, for most micromechanical devices, operation in an inert or stable environment (sometimes vacuum) is a necessity or helps maximize the performance. On the other hand, the ability to integrate micromechanical sensors and actuators with active electronic circuitry is of great interest and is a key step in achieving higher levels of performance and integration in microelectronics.
Many high-performance MEMS devices use silicon dioxide as a sacrificial layer during the fabrication process. The sacrificial dioxide must be removed at the end of the fabrication process to release the device and render it movable and/or functional. The removal of the silicon dioxide is typically carried out in a hydrofluoric acid and de-ionized water (HF/H20) solution, which could also attack and damage passivation and interconnect layer(s) of a CMOS wafer. Therefore, sealing and release techniques that alleviate this problem are of great interest.
Various references discuss MEMS devices and processing methods for producing such devices. These include U.S. Pat. No. 7,023,065 of F. Ayazi et al., issued Apr. 4, 2006, U.S. Pat. No. 6,841,861 of Fredrick T. Brady issued January, 2005, U.S. Pat. No. 6,743,656 of Orcutt, et al., issued June 2004, U.S. Pat. No. 6,469,909 of Simmons, issued October 2002, a paper by S. Pourkamali and F. Ayazi, entitled “High frequency capacitive micromechanical resonators with reduced motional resistance using the HARPSS technology,” proceedings, 5 Silicon RF topical meeting 2004, pp. 147-150, and a paper by S. Pourkamali, Z. Hao and F. Ayazi, entitled “VHF single crystal silicon side supported disk resonators—Part II: implementation and characterization” Journal of Micro Electro Mechanical Systems, Vol. 13, Issue 6, December 2004, pp. 1054-1062. U.S. Pat. No. 5,963,788 of Barron et al. issued Oct. 5, 1999 discloses a method to integrate MEMS devices with CMOS circuits. However, none of these references disclose or suggest a method for encapsulating the MEMS portion of a CMOS wafer, and releasing the wafer for long period of time in HF without damaging the CMOS portion.
It would be desirable to have methods for sealing and backside releasing of microelectromechanical systems with the possibility of providing a vacuum environment to the microelectromechanical device. It would also be desirable to have methods that are suitable for sealing and releasing MEMS integrated with CMOS or microelectronics circuits on a common substrate. It would be desirable to have microelectromechanical devices that are fabricated using the methods.